What does Suncorp host each season? 12 Broncos games, 8 or 9 Dolphins games, Magic weekend, 1-2 origins, a final or 2, a RL test match, some Reds games, 1-2 union internationals, a bunch of Roar games, a couple of soccer internationals or finals. Plus a bunch of concerts and other special events. It really should be 65k to 70k already with that kind of usage.
That said the rest of the money seems like a waste in terms of temporary seating for the athletics track. Is there any reason they can't just knock 10 metres of seating out of the Gabba to fit in the curve?